1. Field of Invention
The present invention relates to a radio frequency (“RF”) combiner/divider capable of automatic impedance transformation for impedance-matching and, more particularly, to a combiner/divider for use in an RF system that includes a changeable number of combiner/divider branches.
2. Related Prior Art
An RF combiner/divider is used to combine several RF signals into a single output RF signal and divide a single RF signal into several output RF signals. The operation of a divider is opposite to that of a combiner. That is, the structure of a divider can be derived from that of a combiner. The combiner combines several input ports into a single output port while the divider divides a single input port into several output ports.
Impedance transformation networks are used in the combiners or dividers. When the characteristic impedance at the input port is not matched with the output impedance at the output port, an impedance transformation circuit increases or reduces the impedance stage between the input and output ports to match the output impedance with the characteristic impedance as much as possible. Impedance-matching is important to ensure the maximum power transformation and minimum signal distortion and/or reflection between input and output circuits.
Korean Patents KR20040069816 and KR20040098857 both describe Wilkinson combiner/dividers based on the Wilkinson Principle. For convenience of description, only the combiners will be discussed for example. Each input branch includes a quarter-wavelength impedance transformer for impedance transformation to match the output impedance with the input impedance. The impedance transformer of each input branch is given limitation. Hence, when the number of the input branches that are combined is changed, the impedance transformer of each input branch must be changed, and this is impractical because such a structure includes a certain number of transformers based on a certain number of channels to be combined, and the impedances of all of the transformers are based on the number of the channels to be combined. Hence, the Wilkinson combiner/dividers based on the Wilkinson Principle are not suitable for systems that include changeable numbers of combined/divided branches.
U.S. Pat. No. 7,046,101 (“'101”) discloses a combiner/divider that is based on the concept of a series/shunt network instead of the Wilkinson Principle. There is disclosed a divider that includes a single-pole N-way RF switch and a switchable impedance-matching network. The switchable impedance-matching network includes N−1 switch-selectable impedance-matching elements. The impedance-matching elements are arranged along a transmission line that includes an input port at an end and a switching connection point at another end. The switching connection point is for selective contact with several output-port reeds. The impedance-matching elements include different impedance-matching lengths. An impedance-matching distance exists between each impedance-matching element and the switching connection point. In operation, when only one output-port reed is in contact with the transmission line, i.e., only one output port is connected to the input port, the load impedance is matched with the source impedance, without having to activate any impedance-matching element. If the number of output ports connected to the input port is changed, the transmission line is connected to an impedance-matching element in a certain position determined by the number of the output ports that are combined, thus initiating an impedance-modulation mechanism for impedance-matching. In practice, the manufacturing and location of the impedance-matching elements require precision.
U.S. Pat. No. 6,323,742 discloses an RF combiner that includes N input channels 126a, 126b, 126c and 126d for receiving input signals. These input channels are electrically connected to an electrical connection point 22 or 132. All of the input signals are combined with one another at the electrical connection point 22 or 132. Then, a quarter-wavelength impedance transformer 34 or 150 transfers the combination of the input signals to an output port. Each input channel includes a grounding switch 26, 28, 30 or 32. There will be high impedance in an input channel if the respective grounding switch is connected to an electrical ground. Hence, the electrical connection point is only connected to an input channel where the grounding switch is open-circuited. An input channel ready for transferring an input signal is defined as an “active input channel.” According to the number of the active input channels, a control circuit 116 controls the connection of a first combiner switch 144 and a second combiner switch 154 to the corresponding impedance transformation line to match the output impedance with the input. The grounding switch provides high impedance to interfere with the ability of the input channels to transfer the signals. That is, the input channels are not actually cut off from the electrical connection point although they cannot smoothly transfer the input signals to the electrical connection point. This practice could easily damage the combiner. Moreover, the structure of the first combiner switch 144 and how it works are not described although it is actually part of an impedance transformer.
The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.